Some of known exhaust purifying devices for purifying nitrogen oxides (NOx) contained in an exhaust gas of a diesel engine include a diesel oxidation catalyst disposed in an exhaust channel of the engine, an ammonia selective catalytic reduction device, a urea-aqueous-solution supply unit for supplying urea aqueous solution to the exhaust gas upstream of the ammonia selective catalytic reduction device, a deposit operation range judgment unit that judges whether an operation of the engine is in a deposit operation range in which urea deposit is deposited in the exhaust channel, a counter that counts the time in which the operation of the engine is in the deposit operation range, and a controller that executes a heatup control for raising the temperature of the exhaust gas in the exhaust channel to a target temperature when the counted time reaches a deposit limit judgment time (see, for instance, Patent Literature 1).
As disclosed in Patent Literature 1, the urea deposit is to be increased in accordance with the operation time of the engine. Accordingly, the controller of Patent Literature 1 supplies fuel into the exhaust channel when a time at which a predetermined amount of the urea deposit is presumably deposited is elapsed to cause a reaction of the fuel at the diesel oxidation catalyst to execute an exhaust heatup control, thereby gasifying and removing the urea deposit.
Further, Patent Literature 1 discloses that an operation status of the engine is judged based on the engine speed and the like per a control cycle, where, when the engine is in the operation status in which it is difficult to raise the temperature of the exhaust gas, the exhaust heatup control is not performed and when the engine is in the operation status capable of raising the temperature of the exhaust gas, the exhaust heatup control is performed.